Along with improvement in performance of superconducting wire and advance in coil manufacturing technique using such wire, as well as technical developments in related apparatus like a heat-insulating container and a refrigerator, various types of superconducting magnets and application apparatus employing such magnets have been created. Among these, there is a type which is operated in a persistent current mode. Superconducting magnet apparatus for a magnetic resonance imaging system (MRI) and for a magnetically levitated vehicle (Maglev) are examples of the type which have already been put into practical use. These superconducting magnet apparatus supply an electric current from an external excitation power source to a coil which is cooled to extremely low temperature. While a necessary magnetic field is produced, initial and final wiring parts of the coil are short-circuited by way of a superconducting switch, and this makes the apparatus run in a persistent current mode in which the electric current continues to flow into the coil without power supply. After the apparatus is switched into the persistent current mode, power from the external excitation power source is turned off and the apparatus is operated independent from the power source. The superconducting magnet apparatus requires a current lead as the component when the electric current is supplied to this coil. The current lead is a current path which couples a terminal connected to the external excitation power source outside the superconducting magnet to an internal coil. From a thermic point of view, the current lead is also a heat leaking path from the terminal at room temperature to the coil at extremely low temperature, and especially when it is not conducted, the lead is merely a heat transmitting member. It is important to minimize heat leaking into a superconducting magnet in order to reduce freezing costs of the coil. Accordingly, it has been considered to use a detachable current lead in the superconducting magnet apparatus operated in the persistent current mode so that the amount of heat leak is reduced by detaching the current lead when the current lead is not conducted.
There are broadly two types of systems for detaching the detachable current lead. One is a system in that an attachment/detachment portion of the current lead is pulled off from the superconducting magnet (see Documents 1 and 4, for example), and the other is a system in that a gap is created at a contact site between the attachment/detachment portion and a fixed portion (lead contact portion) (see Document 2, for example). FIGS. 7A and 7B respectively show an example of a conceptual constitution of the pull-off system and the gap-creating system.
In the pull-off system shown in FIG. 7A, a first current lead 111 is disposed in a vacuum container 110, and a second current lead 112 is configured so that it can be detached from the first current lead 111. The first current lead 111 is connected to a not shown coil inside the vacuum container 110 on one end, and has a lead contact portion 111a on the other end which is exposed to the outside of the vacuum container 110. The second current lead 112 has an attachment/detachment portion 112a on one end for detachably connecting itself to the lead contact portion 111a, and has an electrode terminal 112b on the other end for connecting a lead line which leads to the external excitation power source.
When the external excitation power source supplies the electric current to the coil, the second current lead 112 is inserted to the first current lead 111 for connection, and when the power supply is completed, the second current lead 112 is pulled off from the first current lead 111.
The pull-off system is simple, and thus, a similar constitution has been practiced in the superconducting magnet apparatus for MRI. However, the apparatus employing the pull-off system necessitate professional skills for operation and maintenance of the superconducting magnet, such as in securing a contact pressure at the contact site required for each attachment/detachment upon excitation/demagnetization of the magnet, removal of frost and ice, removal of an insulating coating generated due to oxidization and defacement, or taking measures to prevent the above, etc. The superconducting magnet apparatus according to the pull-off system are not easy to take care of. Therefore, the superconducting magnet apparatus to which such a system can be applied are limited to those, such as the superconducting magnet apparatus for MRI, in which excitation/demagnetization takes place only about once a year and handling of the current lead at the time can be relied on a professional sent for that purpose. Accordingly, in case that there are a plurality of superconducting magnet apparatus of which excitation/demagnetization is performed as needed or every few days, that is, as in the case of the superconducting magnet apparatus for Maglev, if excitation/demagnetization are repeated in a range from every day to every two weeks and a plurality of superconducting magnets installed in one train are continuously excited/demagnetized one after another, manual operation of the detachable current leads will produce an enormous workload. Furthermore, there is also a safety hazard. A strong magnetic force operates on a magnetic body like an iron tool. Under the circumstance that an operator frequently works in the vicinity of a strong magnetic field of the superconducting magnet, there is a fear that the operator may be attracted by the magnet due to the magnetic body the operator accidentally carries.
On the other hand, in the gap-creating system shown in FIG. 7B, a first current lead 121 is disposed inside a vacuum container 120, and a second current lead 122 is designed to be attached to/detached from the first current lead 121. The first current lead 121 is connected to a not shown coil inside the vacuum container 120 on one end, and has a lead contact portion 121a on the other end. The second current lead 122 has an attachment/detachment portion 122a on one end which moves back and forth inside the vacuum container 120 to detachably connect itself to the lead contact portion 121a, and has an electrode terminal 122b on the other end for connecting a lead line leading to the external excitation power source outside the vacuum container 120. Air-tightness inside the vacuum container 120 is maintained by an air-tight lid 125 composed of bellows, etc., disposed in close contact with the vicinity of the attachment/detachment portion 122a to cover a through-hole 120 which the second current lead 122 passes through.
In supplying the electric current to the coil from the external excitation power source, the attachment/detachment portion 122a of the second current lead 122 is connected to the lead contact portion 121a of the first current lead 121. When the supply of the current is completed, the second current lead 122 is moved apart from the first current lead 121, creating a gap between the attachment/detachment portion 122a and the lead contact portion 121a to produce a non-contact state.
This gap-creating system can prevent generation of frost and ice and an insulating coating by providing the contact site between the attachment/detachment portion 122a and the lead contact portion 121a in an air-tight space inside the superconducting magnet. Therefore, operation and maintenance of the superconducting magnet become easy. In case of applying the detachable current lead to the superconducting magnet where excitation/demagnetization of the magnet is comparatively frequent, adoption of this gap-creating system is indispensable.
In the gap-creating system, it is important that the air-tightness at a portion where the vacuum container is pierced is highly reliable. Particularly, in a case of the superconducting magnet used in a dynamic environment which is subject to vibration, a supporting mechanism for ensuring high vibration resistance of the air-tight lid is necessary. Conventionally, only the detachable current lead according to the pull-off system which is simple in design has been practiced. As to the gap-creating system, the detachable current lead which is operated manually without considering such a vibration environment has been proposed (see Document 3, for example).
However, such a manually operated lead has working and safety hazards as well as in the aforementioned lead according to the pull-off system. Also, it is absolutely necessary to apply a required pressing force in order to set contact electric resistance at the contact site equal to or lower than a set value. However, if the operator handles a plurality of detachable current leads very frequently by hand, there may be a shortage of the pressing force due to a human error.
Accordingly, when the detachable current lead is employed in the superconducting magnet of the superconducting magnet apparatus, not only adopting the system of creating a gap at the contact site between the attachment/detachment portion and the lead contact portion but also automation of the operator operation are required. Heretofore, there has been an idea of generating a driving force for the automation by an electric motor. Also, as a sample of only a single detachable current lead portion, there is a disclosure adopting a gas pressure driving system in order to realize the automation (see Document 5, for example).
[Document 1]
Unexamined Japanese Patent Publication No. 61-222209
[Document 2]
Unexamined Japanese Patent Publication No. 60-32374
[Document 3]
Unexamined Japanese Patent Publication No. 3-232205
[Document 4]
Shunji YAMAMOTO, et. al., “Improvement in reliability of a detachable power lead”, Lecture briefs at the 42nd meeting on cryogenic engineering and superconductivity for 1989 Autumn, C1-4, P44 (November, 1989)
[Document 5]
Tsukasa WADA, Akio SATO, “Low heat-leaking detachable power lead”, Resumes for the meeting on cryogenic engineering B3-7, P136 (May, 1987)
However, if a drive unit such as the above electric motor which generates a driving force directly by interaction between an electric current and a magnetic field is disposed in the vacuum container, loss of control or decline in the driving force is caused due to a strong magnetic field generated by the superconducting magnet. In this case, generation of the driving force is principally possible by providing a magnetic shield. However, such a magnetic shield against the strong magnetic field may increase weight of the apparatus and requires a large space. Depending on the design, it is also possible to dispose the drive unit in a vacuum. Then, the air cannot be cooled, and a sufficient amount of electric current cannot be applied in order to restrain heat generation. Consequently, the driving force becomes small and a sufficient amount of contact pressure cannot be applied to the attachment/detachment portion. In other words, a general purpose electric motor which passes a large current for power generation gives off a large amount of heat, thus causing a problem of temperature rise.
In the above gas pressure driving system, it is necessary to connect with pipes and valves a compression and vacuum (decompression) pump, a buffer tank, an expansion/contraction portion for drive (bellows), etc. for reciprocation of the attachment/detachment portion. A number of required components make the design of the apparatus complex and may increase the size and the amount as well. Moreover, if the gas pressure driving system is employed in the superconducting magnet apparatus for Maglev, gas leakage may occur in the pipes, etc. which are susceptible to vibration since traveling vibration is applied while the vehicle is traveling.
One object of the present invention which was made in view of the above problems is to provide a superconducting magnet apparatus, which renders possible efficient, accurate and secure connection of a superconducting magnet current lead.